Cryptococcus neoformans is a pathogenic yeast which most commonly affects the central nervous system and causes fatal meningoencephalitis primarily in patients with AIDS. Considerable progress in the identification and dissection of genes associated with pathogenicity and virulence of Cryptococcus neoformans has been made during the past six years in our laboratory. The most important virulence-associated phenotype in C. neoformans is the formation of a thick extracellular polysaccharide capsule which inhibits phagocytosis by host cells. In previous years, we have cloned and characterized three genes, CAP59, CAP64, CAP60 , which are essential for capsule synthesis and in turn are essential for the organism's ability to cause fatal infection. This year we have assessed several structural and biological properties of an acapsular, serotype unknown strain (602) which was complemented with the CAP64 gene. The chemical structures of the capsular polysaccharide produced in the complemented strain were comprised of triads typical of serotype A. The complemented strains also restored wild type patterns of complement activation and C3 binding, and other immunological findings that point to the role and effect of capsule in virulence and cellular immunity.